Study Finds Abdominal Muscle Contractions Move Fluid in Mouse Brains

Research on mice has found that contractions in abdominal muscles during movement cause the brain to move inside the skull, according to a study published April 27 in Nature Neuroscience. Neuroscientist Patrick Drew, a professor at Penn State University and associate director of the Huck Institutes of the Life Sciences, said imaging has long shown that the brain moves gently inside the fluid-filled skull cavity.

The source of that motion had not been clear. Drew's team used advanced imaging to observe mouse brains before and after the animals began walking. The brain moved just milliseconds before a mouse took a step, at the moment when abdominal muscles contracted in preparation for movement.

Researchers then applied slight pressure to the abdominal muscles of lightly anesthetized mice using pressure sensors. The same brain motion occurred. Breathing or cardiac activity did not produce the same response. The connection is the vertebral venous plexus, a network of veins that links the abdomen to the spine in both mice and humans.

"It's like a hydraulic system. It really is very much like the jacks that push your car up, or something that an excavator might have," Drew said. "Whenever you tense those muscles, which you do whenever you make a movement . . . " Computer simulations run by the team showed that the contractions generated by walking move cerebrospinal fluid out of the brain.

Drew said the mechanism may play a role in flushing excess proteins and other unnecessary material from brain tissue. "It's more speculative, but using simulations, we can see that this sort of motion should drive fluid movement and could help clear waste in the brain," Drew said.

Future research will examine whether the brain detects these mechanical signals and how conditions such as obesity affect the relationship between abdominal muscles and the brain. Michael Goard, an associate professor at UC Santa Barbara who studies sensory and spatial processing, said the work thoroughly identified the mechanical causes of the movement during locomotion.

The findings clarify a long-observed cerebral movement and its link to physical activity. They suggest a basic physiological process that could apply to broader research on brain waste clearance. The study does not address whether the same mechanism operates identically in humans.